Such a conventionally known type of activated carbon for an electrode includes alkali-activated carbon, which is a carbonized powder made using a graphitizable carbon powder, e.g., mesophase pitch as a starting material, aiming at the increase of the electrostatic capacity.
This alkali-activated carbon is produced by a process comprising a step of producing a fibrous material by spinning using mesophase pitch, a step of subjecting the fibrous material to an infusibilizing treatment and then to a carbonizing treatment, a step of subjecting the carbonized material to an alkali activating treatment and then to a pulverizing treatment, or to a pulverizing treatment and then to an alkali activating treatment.
The alkali-activated carbon made in the conventional process, however, suffers from the following problem: The alkali-activated carbon is an activated carbon powder made by pulverizing the fibrous material and hence, even if the length of particles of the powder is shortened due to the pulverization, the longitudinal breaking of the particles, namely, the breaking such as to break end faces of the fiber is hard to occur, and the over-pulverization brings about the degradation of the performance. Thus, the alkali-activated carbon contains a large number of columnar particles. If an electrode is produced using such alkali-activated carbon, the columnar particles are dispersed at random, whereby gaps are liable to be created between the columnar particles. As a result, an electrode density (g/cc) is low and it is impossible to increase the electrostatic capacity density (F/cc) of an electric double-layer capacitor.
If the alkali acting treatment is utilized, it is possible to produce activated carbon for an electrode, which has relatively uniform pores made therein and a high electrostatic capacity density. However, the activated carbon for the electrode having the pores made by the alkali activation with such an electrostatic capacity density being largely taken into account is accompanied by a problem that it is difficult to ensure a pore diameter sufficient for the diffusion of a liquid electrolyte and ions, and an electric double-layer capacitor produced using the activated carbon and the like has an increased internal resistance due to the foregoing.
Further, if a polarizing electrode is formed using the activated carbon for an electrode produced in the conventional process, the amount of polarizing electrode expanded during charging is large. For this reason, for example, in a laminated-type or rolled-type electric double-layer capacitor, it is necessary to take a measure for providing a space corresponding to the amount of polarizing electrode expanded within a case, or a measure for increasing the strength of the case to receive a force of expansion of the polarizing electrode. However, the former brings about a disadvantage of a decrease in electrostatic capacity per unit volume, and the latter brings about disadvantages of an increase in cost of the case, an increase in weight of the case and the like.